As is widely known, a fuel cell system has been proposed and placed in practical use to generate electricity by putting a hydrogen-rich fuel gas and an oxygen-containing gas which may be the air to the electrode reaction. A representative example of the fuel cell system includes a generation chamber, a plurality of fuel cells arranged in the generation chamber, fuel gas feeding means for feeding a fuel gas to the fuel cells, oxygen-containing gas feeding means for feeding an oxygen-containing gas to the fuel cells, and power converter means for converting a DC output of the fuel cells into an alternating current. The power converter means feeds AC power to the load such as electric appliances in a general household in cooperation with a system power source such as a commercial power source. The fuel cell system further includes power control means for suitably controlling the amount of electric power output to the power converter means from the fuel cells depending upon variations in the load, and generation control means for controlling the generation of the fuel cells by controlling the flow rate of the fuel gas and the flow rate of the oxygen-containing gas fed to the fuel cells. It is desired that the amount of generation of the fuel cells is suitably controlled depending upon variations in the load. Accordingly, the power control means and the generation control means work in cooperation with each other.
When the load increases and, hence, the amount of output power of the fuel cells must be increased, the amount of output power is instantaneously increased in response to an increase instruction involving, however, a time lag of several milliseconds. On the other hand, the fuel gas to be fed to the fuel cells is obtained from a fuel gas to be reformed, such as a city gas that must be reformed into a hydrogen-rich fuel gas. Therefore, the flow rate of the fuel gas cannot be instantaneously increased despite of having received the increase instruction, and increases involving a time lag of several seconds. Therefore, when the amount of output power must be increased at a large rate, the fuel gas becomes in short supply in the fuel cells giving rise to the occurrence of a so-called fuel depletion phenomenon. If the fuel depletion phenomenon occurs, the electric power is not obtained as required and, besides, the fuel cells are deteriorated.
To avoid the occurrence of the above fuel depletion phenomenon, JP-A-7-14598 discloses a fuel cell system wherein when the load has increased, the flow rate of the fuel gas and the flow rate of the oxygen-containing gas are readily increased in response thereto, but the amount of the output power is increased being suitably lagged behind. Further, JP-A-7-57753 discloses a fuel cell system wherein when the load has increased, the rate of increasing the output power is limited to be not larger than a predetermined value to thereby avoid the occurrence of the fuel depletion phenomenon.
However, the following problems are involved in the conventional fuel cell systems which are designed to avoid the occurrence of the fuel depletion phenomenon as described above. Namely, the rate of following up the variations in the load becomes relatively sluggish since the output power is increased in a delayed manner or the rate of increasing the output power is limited to be not larger than a predetermined value. When the load varies vigorously, the electric power in many cases is generated without properly following the variation in the load, and the effective generation efficiency decreases considerably. Further, controlling the output power by the power control means must be suitably related to controlling the flow rate of the fuel gas and the flow rate of the oxygen-containing gas by the generation control means, and it becomes necessary to execute these control operations systematically. Therefore, when it is desired to change the load for which the fuel cell system is to be provided or when the fuel cell system is to be added to a single load, the control system as a whole must be greatly modified.